1. Field of the Invention
This invention relates to a process for preparing a vinyl chloride polymer, particularly to a process for preparing a vinyl polymer which is free from variance in take-up of plasticizer of the polymer obtained, and can obtain a homogeneous vinyl chloride polymer stably.
2. Description of the Prior Art
In the prior art, in preparation of a vinyl chloride polymer by polymerization of vinyl chloride or a monomeric mixture composed mainly of vinyl chloride in an aqueous medium, for obtaining a polymer of constant quality at all times, there has been proposed the method of terminating the polymerization by addition of an organic solvent solution of an antioxidant at a predetermined stage at the last stage of polymerization. However, in this method, the antioxidant cannot be uniformly dispersed in the polymerization system, and it has been difficult to terminate completely the polymerization over the entire region of the polymerization system. For this reason, even if the antioxidant may be added at the same timing, the quality of the polymer, for example, take-up of plasticizer differed from batch to batch to involve variance, whereby no homogeneous polymer could be prepared stably. Accordingly, for the purpose of dispersing the antioxidant uniformly into the polymerization system, there have been proposed the method of adding an aqueous dispersion of an antioxidant using an emulsifier into the polymerization system (Japanese Pre-examination Patent Publication (KOKAI) Nos. 149608/1985 and 186547/1985) and the method of adding an aqueous dispersion of an antioxidant using a water-soluble partially saponified polyvinyl alcohol into the polymerization system (Japanese Pre-examination Patent Publication (KOKAI) No. 218711/1983).
However, in the above-mentioned method using an emulsifier, initial coloration characteristic and thermal stability of the polymer obtained are worsened. In the method using a water-soluble partially saponified polyvinyl alcohol, there is involved the problem that the COD value of the wastewater from the polymerization vessel becomes high, etc.